In dogs anaesthetized with chloralose, small latex balloons were positioned at the left pulmonary vein-atrial junctions so as to stretch this region. By recording action potentials from slips of the cervical vagi it was established that distension of these balloons stimulated receptor endings in the atrial endocardium which discharged into the myelinated branches of the vagi i.e. Paintal type A, type B and Intermediate type receptors. 2. In other dogs, cooling the cervical vagus in steps of 2 degrees C reduced this response in vagal myelinated fibres. With twelve receptors the response to distension was reduced by 30% when the vagus was cooled to 16 degrees C, by 70% when cooled to 12 degrees C and abolished at 8--12 degrees C. 3. In a third group of dogs, distension of balloons at the pulmonary vein-atrial junctions resulted in a reflex increase in heart rate. Cooling the cervical vagi in these dogs in stages to 8 degrees C reduced this increase in heart rate. In nine dogs the response was reduced by 20% when the vagi was cooled to 16 degrees C, by 70% when cooled to 12 degrees C and abolished between 12 and 8 degrees C. 4. In a fourth group of dogs, distension of balloons at the pulmonary vein-atrial junctions was shown also to activate receptor endings in the atria which discharged into non-myelinated branches of the Vagi. In twelve receptors, cooling the cervical vagus in steps of 2 degrees C reduced this evoked increase in activity in non-myelinated fibres. These responses were abolished over a wide range of temperature unlike the responses observed above. 5. It is concluded that the increase in heart rate which follows distension of balloons at the pulmonary vein-atrial junctions is mediated solely by the Paintal-type receptors which discharge into the myelinated fibres in the vagi.
